Quantum computing hurdle overcome

Emma Woollacott, 25th June 2009

Ann Arbor, Michigan - Physicists have overcome one of the major hurdles to quantum computing by finding a way to increase the shelf life of quantum bits by 1,000 times.

Quantum bits - formed in this case by arrays of semiconductor quantum dots containing a single extra electron - have the worrying habit of forgetting the information they're supposed to be storing. This is because they are easily perturbed by magnetic field fluctuations from the nuclei of the atoms creating the quantum dot.

The scientists used lasers to elicit a previously undiscovered natural feedback reaction that stabilizes the quantum dot's magnetic field, lengthening the stable existence of the quantum bit by more than 1,000 times.

"In our approach, the quantum bit for information storage is an electron spin confined to a single dot in a semiconductor like indium arsenide. Rather than representing a 0 or a 1 as a transistor does in a classical computer, a quantum bit can be a linear combination of 0 and 1," said Duncan Steel, a professor in the University of Michigan's Department of Physics. "One of the serious problems in quantum computing is that anything that disturbs the phase of one of these spins relative to the other causes a loss of coherence and destroys the information that was stored."

A major cause of information loss in a popular class of semiconductors called 3/5 materials is the interaction of the electron - the quantum bit - with the nuclei of the atoms in the quantum dot holding the electron. Trapping the electron in a particular spin, as is necessary in quantum computers, gives rise to a small magnetic field that couples with the magnetic field in the nuclei and breaks down the memory in a few billionths of a second.

By exciting the quantum dot with a laser, the scientists were able to block the interaction of these magnetic fields. The laser causes an electron in the quantum dot to jump to a higher energy level, leaving behind a charged hole in the electron cloud. This hole also has a magnetic field due to the collective spin of the remaining electron cloud. It turns out that the hole acts directly with the nuclei and controls its magnetic field without any outside intervention except the excitation by the lasers.

"This discovery was quite unexpected," Steel said. "We still have other major technical obstacles, but our work shows that one of the major hurdles to quantum computers that we thought might be a show-stopper isn't one," Steel said.